Object.keys

Object.keys(obj)

var arr = ['a', 'b', 'c'];
console.log(Object.keys(arr) // console: ['0', '1', '2']

// array like object
var obj = { 0: 'a', 1: 'b', 2: 'c' };
console.log(Object.keys(obj) // console: ['0', '1', '2']

// array like object with random key ordering
var anObj = { 100: 'a', 2: 'b', 7: 'c' };
console.log(Object.keys(anObj) // ['2', '7', '100']

// getFoo is property which isn't enumerable
var myObj = Object.create{}, {
  getFoo: {
    value: function () { return this.foo; }
  } 
}
myObj.foo = 1;
console.log(Object.keys(myObj) // console: ['foo']

Object.keys('foo'
// TypeError: "foo" is not an object (ES5 code)

Object.keys('foo'
// ["0", "1", "2"]                   (ES2015 code)

// From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
if (!Object.keys) {
  Object.keys = (function() {
    'use strict';
    var hasOwnProperty = Object.prototype.hasOwnProperty,
        hasDontEnumBug = !{ toString: null }).propertyIsEnumerable('toString'),
        dontEnums = [
          'toString',
          'toLocaleString',
          'valueOf',
          'hasOwnProperty',
          'isPrototypeOf',
          'propertyIsEnumerable',
          'constructor'
        ],
        dontEnumsLength = dontEnums.length;

    return function(obj) {
      if (typeof obj !== 'function' && (typeof obj !== 'object' || obj === null)) {
        throw new TypeError('Object.keys called on non-object'
      }

      var result = [], prop, i;

      for (prop in obj) {
        if (hasOwnProperty.call(obj, prop)) {
          result.push(prop
        }
      }

      if (hasDontEnumBug) {
        for (i = 0; i < dontEnumsLength; i++) {
          if (hasOwnProperty.call(obj, dontEnums[i])) {
            result.push(dontEnums[i]
          }
        }
      }
      return result;
    };
  }()
}